While rats are often brought to the clinic after developing an infectious or parasitic disease, the underlying cause frequently lies in poor husbandry. As long as the animal’s living conditions are not changed, the illness persists or comes back.
Classification
| Class: | Mammals |
| Order: | Rodents |
| Suborder: | Sciurognaths |
| Family: | Muridae |
| Subfamily: | Murinae |
| Genus: | Rattus |
| Species: | norvegicus |
| Common name: | pet rat, domestic rat |
Biological and physiological features
- Adult male weight: 450–500 g.
- Adult female weight: 250–300 g.
- Total length: 35–50 cm.
- Tail length: 17–23 cm.
- 4 toes on the forelimbs and 5 toes on the hindlimbs.
- Dental formula:
| Upper jaw | 1 | 0 | 0 | 3 |
| I | C | PM | M | |
| Lower jaw | 1 | 0 | 0 | 3 |
Biological parameters
- The rat is active in mixed bouts, but mainly at dusk and at night. It is active all year round. It can gnaw through very hard materials. In the wild, the rat digs burrows with several entrances in the banks of watercourses.
- The rat’s social system is made up of hierarchical family groups. Keeping one on its own is prohibited.
- The rat does not hibernate.
- It lives 2 to 3 years and has a body temperature of 36–37.5°C.
Reproductive physiology
The male and female are sexually mature at 60 days, and the breeding season lasts all year round.The female’s cycle lasts 4 to 5 days, and her ovulation is spontaneous.
Pregnancy is 20 to 23 days. There can be 6 to 12 young per litter, and the female can have 4 to 5 litters a year.
At birth the babies have their eyes closed, then open them at 13 to 15 days of age. They weigh 5–6 g. The young should be weaned at 21 days of age. Just after the birth, it is best not to touch the female, as she might eat her offspring.
Beware: you must separate the parents before the birth, otherwise in less than 12 hours afterwards the pair are already able to breed again.
Feeding
The rat is an omnivorous rodent that eats 30–40 g of food a day. A shop-bought complete rat mix with 20–27% protein suits it well. You can also give it a home-made ration of seeds, green vegetables, carrots, apples and a little meat; but beware — the rat is prone to obesity, so its ration must be limited.
Living environment
The cage must be spacious — 100×50×70 cm for 2 rats, but ideally, for 2 to 4 rats, allow 2 m² of floor space and 2 m of height. You can use an aviary-style enclosure with several levels, or a cabinet. No terrarium. For bedding, hemp litter works well, or beech shavings or corn-cob litter. The bedding should be changed twice a week. Provide various levels, hiding places, spots to retreat to, ladders and branches, including some to gnaw (hazel, beech, untreated fruit trees). Ideally the cage should be set up in a room where the room temperature is 18–26°C and the humidity is between 40 and 70%.
Handling
To hold a rat properly, support it with one hand on the skin between the ears and the other hand at the base of the tail. Avoid the tip of the tail, which may otherwise slip through your fingers.
Common conditions
Respiratory disorders
Infectious respiratory disorders are the most common problems in the rat. Two major clinical syndromes may be encountered:
- Chronic respiratory disease: it is caused by a bacterium called Mycoplasma pulmonis, whose disease-causing power is heightened in the presence of a virus — the Sendai virus, or the respiratory syncytial virus. The clinical signs are telling: breathing difficulty made worse by exertion, weight loss, nasal discharge (secretion running from the nose), nosebleeds. A number of factors encourage the disease to develop: an ammonia level that is too high (it should be below 10 ppm), a genetic predisposition, a lack of vitamins A and E, and then the presence of the Sendai virus and of the respiratory syncytial virus. Generally the treatment the vet gives does not completely eliminate the pathogen, so it will have to be repeated.
- Pneumonia: it is caused by a streptococcus. Its onset is sudden and severe, and sudden deaths are common. Adults show breathing difficulty, abdominal breathing and a purulent nasal discharge that soils the nostrils and paws. Besides streptococci, there is a second causative agent in cases of immunosuppression (tumour, diabetes, nutritional deficiencies): corynebacterium kutscheri. Diagnosis rests on the clinical signs and on identifying the germ by culture.
Tumours
The consequences of tumours vary with age, sex, diet and general condition, and depend on endocrine and environmental factors.
Common conditions
By epidemiology:
- Mammary tumours between 12 and 24 months
- Haemato-lymphopoietic tumours after 2 years
- Ovarian and testicular tumours are uncommon, apart from Leydig cell tumours.
Predisposing factors:
- Illnesses and degenerative processes linked to rapid growth and higher body weight.
- Isolation encourages the development of tumours in rodents.
- The excessive calcium and phosphorus but deficient magnesium content of commercial rodent diets is thought to bring on urinary tumours indirectly.
- Chronic viral or bacterial infections are thought to encourage the development of tumours of the haemato-lymphopoietic tissues in rodents.
Common tumours in males:
- Males mainly get tumours of the endocrine glands
- Pituitary tumours
- Mammary tumours, which are common in the male and are morphologically identical to those seen in the female.
- Leydig cell tumours, which are common, especially with age, bilateral, and benign in 90% of cases.
Common tumours in females:
- The tumour rate is higher in the female.
- Mammary fibroadenomas
- Uterine polyps, which can sometimes cause vaginal bleeding.
- Pituitary adenomas, which may be favoured by a diet too rich in protein; non-secreting and well-circumscribed, they can press on the inside of the skull, causing nervous and sometimes respiratory disturbances.
Mammary tumours:
Mammary growth is hormone-dependent: increasing development after several cycles, then a senile regression during the second year. Mammary tumours come in 3 different types:
- Lobular hyperplasia lesions, which are common. They can reach 8–10 centimetres across. They are forerunners of mammary tumours.
- Fibroepithelial tumours, or fibroadenomas, are also common (26%), particularly in the Sprague Dawley. They are well-defined, firm, oval or round, and benign.
- Adenocarcinomas are rare (10%). They are marked by haemorrhagic, ulcerative and necrotic lesions.
Because mammary tissue extends so widely, mammary tumours can be seen in almost any area under the skin. Surgical removal gives a good prognosis if the tumour is benign. The prevalence of mammary tumours is lower in spayed females. Fibroadenoma recurrences are nonetheless common.
Tumours common to both sexes:
A number of tumours affect both males and females.
- Skin tumours: fibromas, fibrosarcomas and lymphomas.
- Tumours of the haematopoietic system: malignant lymphomas, myeloid leukaemias, histiocytic sarcomas, fibrosarcomas.
- Thyroid tumours: adenomas.
- Tumours of the peripheral nervous system: most often these are only histological findings.
Skin disorders
- Mange, with hair loss, itching and crusts
- Pseudo-mange, with localised crusty lesions
- Fungal infections, with rounded patches of hair loss
- Bacterial ulcerative dermatitis
- Dry gangrene of the tail
Digestive disorders
- Dental malocclusion
- Sialodacryoadenitis (viral)
Urinary disorders
- Chronic nephritis
Eye disorders
- Chromodacryorrhoea, or red tears, often linked to stress
Zoonoses
- Fungal infections (ringworm)
- Tapeworm (Taenia)
- Lymphocytic choriomeningitis